Implantable medical devices (IMDs) can include cardiac function management (CFM) devices such as implantable pacemakers, implantable cardioverter defibrillators (ICDs), cardiac resynchronization therapy devices (CRTs), and devices that include a combination of such capabilities. The devices can be used to treat patients or subjects using electrical or other therapy, or to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition. The devices may include one or more electrodes in communication with one or more sense amplifiers to monitor electrical heart activity within a patient, and often include one or more sensors to monitor one or more other internal patient parameters. The sense amplifiers may be included in an electronics unit that performs diagnostic functions and functions related to delivery of therapy. IMDs can also include neural stimulation devices to provide electrical stimulation or other therapy to a portion of the nervous system of the patient. The electronics unit often includes microcontrollers or microprocessors along with memory to store information such as program instructions and data. The present inventors have recognized a need for improved reliability and improved design flexibility of IMDs.